Diaphragm valves provide excellent sealing and isolation characteristics to contain fluid being controlled and prevent migration of the controlled fluid into the valve mechanisms or out of the valve. Diaphragm valves utilize a resilient diaphragm and a valve shoulder to engage a valve seat and prevent the flow of fluid past the valve seat. A weir-type diaphragm valve is a valve utilizing a resilient diaphragm that engages a weir to control flow of fluid over the weir. The diaphragm may be controllably lifted and sealed against the weir to selectively permit flow through the valve.
Weir-type diaphragm valves are often employed in the biotechnological, pharmaceutical, chemical, food processing, beverage, cosmetic, and semiconductor industries. These industries often require valves that protect against product contamination and leakage within the valve, workplace and atmosphere. Weir-type diaphragm valves are well suited to meet these requirements because the mechanical valve parts are isolated from fluid flowing through the valve.
Traditionally, diaphragm valves were made of metal alloys. Such metal valves provide good durability and service life in basic fluid control applications. However, metal alloys are not well suited to some process environments, such as pharmaceutical and semiconductor manufacturing. In those applications, the fluids often used are highly corrosive or caustic and also must be kept ultra pure. These corrosive fluids can erode the metal from the valve body and contaminate the ultra pure process fluids. Also, some metal alloys may act as catalysts causing the process fluids to undergo chemical reactions, thereby compromising end products, and potentially, worker safety.
Specialized high strength alloys and stainless steels have been developed to minimize reactivity and erosion in the valve bodies. However, such specialized alloys are very difficult to cast or machine into valve components. The resulting valves are very costly to purchase relative to traditional metal valves. Moreover, stainless steel is not suitable in particular applications such as the semiconductor processing industry.
Plastic lined metal valves were developed to allow traditional metal valve bodies to handle caustic fluids in specialized process applications. The metal valve body is first formed by casting or machining. Then, a plastic or fluoropolymer is molded in the interior of the valve body where process fluids contact the body. U.S. Pat. No. 4,538,638 discloses a plastic lined metal bodied diaphragm valve.
Although, the plastic lined metal valves and plastic lined plastic valves may provide the desired resistance to degradation by process fluids, manufacturing costs are high. High costs are attributable to the complicated multi-step manufacturing process of molding a plastic lining in a support body. The plastic lining may be subject to creep with respect to its surface underneath. Creep reduces the useful life of the expensive plastic lined valve.
Through advances in plastics and manufacturing technologies, valves made entirely or almost entirely of fluoropolymers have become commercially viable. Such plastic valves are capable of providing a cost effective valve having desirable non-reactive and corrosive resistant properties ideally suited for use in pharmaceutical and semiconductor manufacturing applications. U.S. Pat. Nos. 5,279,328 and 4,977,929 disclose plastic diaphragm valves. In certain applications, plastic bodied valves may also be provided with a plastic lining. U.S. Pat. No. 4,538,638 discloses a plastic lined diaphragm valve.
While fluoropolymer valves and plastic valves having fluoropolymer liners are well suited to withstanding caustic fluids, they are susceptible to dimensional degradation such as warpage and creep. Fluids used in industrial processes, such as the pharmaceutical, sanitary, and semiconductor industries, generally require the process fluids to be kept ultra pure. Components used in fluid delivery systems, such as valves, are routinely cleansed to ensure that contaminants do not become trapped in such components and thereby introduced into the process system.
The cleansing processes may involve exposure to high temperature steam for a sufficient amount of time to sterilize the component. Particularly when repeated numerous times, this sterilization process can cause the plastic in the valve to change dimension slightly, resulting in warpage. Creeping results when plastic is subject to stress over a period of time. The plastic component's dimensions can change from the stress. Due to such warpage and creep, tolerances, especially at the weir, are affected and leakage may result. Therefore, a need exists to provide a plastic lined weir-type diaphragm valve that has improved dimensional stability when exposed to repeated cleansing operations or exposure to conditions normally conducive to warpage or creep.